If the answers are same for Oracle, MySQL and SQL databases, only one query is specified. If the SQL Commands differ for Oracle, MySQL and SQL Server, specific queries with respect to a database is provided.

Interview Questions on SQL are based on following two tables, Employee Table and Employee Incentive Table.

Get names of employees from employee table who has '%' in Last_Name. Tip : Escape character for special characters in a query.

SQL Queries in Oracle, Select FIRST_NAME from employee where Last_Name like '%?%%'
SQL Queries in SQL Server, Select FIRST_NAME from employee where Last_Name like '%[%]%'
SQL Queries in MySQL, Select FIRST_NAME from employee where Last_Name like '%\%%'

Get Last Name from employee table after replacing special character with white space

SQL Queries in Oracle, select min(salary) from (select * from (select * from employee order by SALARY desc) where rownum < 3)
SQL Queries in SQL Server, select min(SALARY) from (select top 2 * from employee) a
SQL Queries in MySQL, select min(SALARY) from (select * from employee order by salary desc limit 2) a

Select Nth Highest salary from employee table

SQL Queries in Oracle, select min(salary) from (select * from (select * from employee order by SALARY desc) where rownum < N + 1)
SQL Queries in SQL Server, select min(SALARY) from (select top N * from employee) a
SQL Queries in MySQL, select min(SALARY) from (select * from employee order by salary desc limit N) a

SQL Queries Interview Questions and Answers on "SQL Union" - Examples

Select First_Name,LAST_NAME from employee table as separate rows

select FIRST_NAME from EMPLOYEE union select LAST_NAME from EMPLOYEE

What is the difference between UNION and UNION ALL ?

Both UNION and UNION ALL is used to select information from structurally similar tables. That means corresponding columns specified in the union should have same data type. For example, in the above query, if FIRST_NAME is DOUBLE and LAST_NAME is STRING above query wont work. Since the data type of both the columns are VARCHAR, union is made possible. Difference between UNION and UNION ALL is that , UNION query return only distinct values.

Explanation : Here exists statement helps us to do the job of If statement. Main query will get executed if the sub query returns at least one row. So we can consider the sub query as "If condition" and the main query as "code block" inside the If condition. We can use any SQL commands (Joins, Group By , having etc) in sub query. This command will be useful in queries which need to detect an event and do some activity.

How to fetch data that are common in two query results ?

select * from EMPLOYEE where EMPLOYEE_ID INTERSECT select * from EMPLOYEE where EMPLOYEE_ID < 4
Explanation : Here INTERSECT command is used to fetch data that are common in 2 queries. In this example, we had taken EMPLOYEE table in both the queries.We can apply INTERSECT command on different tables. The result of the above query will return employee details of "ROY" because, employee id of ROY is 3, and both query results have the information about ROY.

Get Employee ID's of those employees who didn't receive incentives without using sub query ?

select EMPLOYEE_ID from EMPLOYEE
MINUS
select EMPLOYEE_REF_ID from INCENTIVES
Explanation : To filter out certain information we use MINUS command. What MINUS Command odes is that, it returns all the results from the first query, that are not part of the second query. In our example, first three employees received the incentives. So query will return employee id's 4 to 8.

Select 20 % of salary from John , 10% of Salary for Roy and for other 15 % of salary from employee table

SELECT FIRST_NAME, CASE FIRST_NAME WHEN 'John' THEN SALARY * .2 WHEN 'Roy' THEN SALARY * .10 ELSE SALARY * .15 END "Deduced_Amount" FROM EMPLOYEE
Explanation : Here we are using SQL CASE statement to achieve the desired results. After case statement, we had to specify the column on which filtering is applied. In our case it is "FIRST_NAME". And in then condition, specify the name of filter like John, Roy etc. To handle conditions outside our filter, use else block where every one other than John and Roy enters.

SQL Queries in Oracle, SELECT distinct DECODE (DEPARTMENT, 'Banking', 'Bank Dept', 'Insurance', 'Insurance Dept', 'Services', 'Services Dept') FROM EMPLOYEE
SQL Queries in SQL Server and MySQL, SELECT case DEPARTMENT when 'Banking' then 'Bank Dept' when 'Insurance' then 'Insurance Dept' when 'Services' then 'Services Dept' end FROM EMPLOYEEExplanation : Here DECODE keyword is used to specify the alias name. In oracle we had specify, Column Name followed by Actual Name and Alias Name as arguments. In SQL Server and MySQL, we can use the earlier switch case statements for alias names.

delete from EMPLOYEE where EMPLOYEE_ID in (select EMPLOYEE_REF_ID from INCENTIVES)

Explanation : Trick about this question is that we can't delete data from a table based on some condition in another table by joining them. Here to delete multiple entries from EMPLOYEE table, we need to use Subquery. Entries will get deleted based on the result of Subquery.

Insert into employee table Last Name with " ' " (Single Quote - Special Character)

Tip - Use another single quote before special character
Insert into employee (LAST_NAME) values ('Test''')

Select Last Name from employee table which contain only numbers

Select * from EMPLOYEE where lower(LAST_NAME) = upper(LAST_NAME)Explanation : Here in order to achieve the desired result, we use ASCII property of the database. If we get results for a column using Lower and Upper commands, ASCII of both results will be same for numbers. If there is any alphabets in the column, results will differ.

Write a query to rank employees based on their incentives for a month

select FIRST_NAME,INCENTIVE_AMOUNT,DENSE_RANK() OVER (PARTITION BY INCENTIVE_DATE ORDER BY INCENTIVE_AMOUNT DESC) AS Rank from EMPLOYEE a, INCENTIVES b where a.EMPLOYEE_ID = b.EMPLOYEE_REF_IDExplanation : Here in order to rank employees based on their rank for a month,DENSE_RANK keyword is used. Here partition by keyword helps us to sort the column with which filtering is done. Rank is provided to the column specified in the order by statement. The above query ranks employeeswith respect to their incentives for a given month.

Update incentive table where employee name is 'John'

Explanation : Here we need to join Employee and Incentive Table for updating the incentive amount. But for update statement joining query wont work. We need to use sub query to update the data in the incentive table. SQL Query is as shown below.